#!/usr/local/bin/perl
#!/usr/bin/perl
use strict;
use warnings;

package Monad::IO;

sub new {
    my $class = shift;
    bless shift, $class;
}

sub bind {
    my ( $m0, $fm1 ) = @_;
    $fm1->( $m0->() );
}

sub push {
    my ( $m0, $m1 ) = @_;
    shift->bind( __PACKAGE__->new( sub { $m1 } ) );
}

use overload
  '>>='    => sub { shift->bind(@_) },
  '>>'     => sub { shift->push(@_) },
  fallback => 1,
  ;

# export の代わり
sub main::monad(&) { Monad::IO->new(shift) }

package main;

my $getClockTime = monad { localtime() . '' };
my $putStrLn = sub {
    my $str = shift;
    monad { print "$str\n" };
};

sub pi { atan2( 1, 1 ) * 4 }
# >>= も >> も perl では右結合なので括弧
my $main = ( $getClockTime >>= $putStrLn ) >> $putStrLn->( pi() );

$main->();